This invention relates to a method of producing hydrogenated amorphous silicon which comprises thermally decomposing a silane gas by directing a stream of said silane gas to impinge on a surface such as a tungsten or carbon foil heated to about 1400.degree.-1600.degree. C. in a vacuum to form a mixture of atomic hydrogen and atomic silicon, depositing said mixture onto a substrate, which can be heated to a temperature of less than 500.degree. C., situated above said foil, and recovering hydrogenated amorphous silicon. Initial measurements indicate that the resulting films have a fairly high photoresponse. The introduction of ammonia along with the silane enhances the photoconductivity of the resultant hydrogenated amorphous silicon films.
Amorphous silicon is a material which has commercial application as a low cost photovoltaic material. However, the methods heretofore utilized in its preparation have limited its utility because of the presence of impurities and the failure of the removal of such impurities from the deposited amorphous silicon; as well as the non-uniformity in the electrical properties of the resultant deposited amorphous silicon film. This may have detrimental effects on electrical performance.
In addition, presently available processes of producing amorphous silicon, such as electron beam deposition, cause clusters of silicon to form, which are believed to degrade the properties of amorphous silicon.
It has now been found that in the present process, operation at high vacuum prevents interactions in the gas phase avoiding silicon cluster formation due to the long mean free paths. In addition, control of the temperature of the tungsten or graphoil surface will control the decomposition compounds coming from the silane gas or other silicohydride gas. Another advantage of present process, which utilizes a high vacuum environment, is that it can be used in combination with electron beam deposition.
The hydrogenated amorphous silicon produced by present process will possess superior properties since the silicon will be purely atomic prior to deposition. It is further believed that the excellent properties of this material comes from the significant passivation of dangling (free) bonds present in pure amorphous silicon, with the atomic hydrogen that results from the silane decomposition.
Accordingly, it is a primary object of instant invention to provide hydrogenated amorphous silicon having particular utility as a photovoltaic material, and for other uses involving semiconductors.
Another object of instant invention is to provide a process for the thermal decomposition of a silane from a hot tungsten or graphoil surface, into a gaseous mixture of atomic hydrogen and atomic silicon which is deposited on a substrate separate from and outside the source of thermal decomposition to form hydrogenated amorphous silicon.
Still another object of instant invention is to provide a hydrogenated amorphous silicon of improved photoconductivity and uniform electrical properties.
Still another object of instant invention is to provide an efficient process for producing a hydrogenated amorphous silicon film of controlled electrical properties.
Accordingly, present invention relates to a process of producing hydrogenated amorphous silicon which comprises thermally decomposing a gas containing silicon and hydrogen such as silane, disilane, trisilane, tetrasilane and the like into a mixture of atomic hydrogen and atomic silicon, and depositing said gaseous mixture onto a substrate separate from the source of thermal decomposition, which may or may not be heated.
More specifically, the present process of producing hydrogenated amorphous silicon comprises decomposing a silane gas by directing a beam of silane gas, preferably in an ammonia atmosphere, at a tungsten or graphoil sheet heated to a temperature of about 1400.degree.-1600.degree. C., in a vacuum of about 10.sup.-6 to 10.sup.-4 torr, into a gaseous mixture of silicon and atomic hydrogen, and collecting the reaction products on a substrate mounted above the heated sheet. This process involves a single, possibly catalyzed interaction between a silane molecule and a hot tungsten or graphoil surface to produce atomic silicon and atomic hydrogen. Upon hitting the hot foil, a portion of the silane gas (SiH.sub.4) dissociates into a mixture of Si, H, SiH, SiH.sub.2 and SiH.sub.3. The relative proportion of the products is a function of the foil temperature. By directing a stream of silane (from a copper tube) at the hot foil, a locally high silane pressure is created at the surface of the hot foil (on the order of about 4.times.10.sup.-4 torr). The decomposition takes place at the surface, such that a high pressure at the hot surface is necessary for the efficient operation of this method. This process allows for a more sensitive control of the local pressure at the hot foil, making for an efficient deposition process. Use of high vacuum makes this process compatible with other high vacuum techniques, such as electron beam deposition. Present process provides for a simple thermal decomposition process compared to glow discharge decomposition of silane in the production of amorphous silicon.